194
Marine Mammal Physiology: Requisites for Ocean Living
versus dissipate heat varies widely given the global distribution of this large group of
mammals. For instance, some cetaceans spend their entire lives in ice-laden polar waters,
and therefore would only need to disperse excess heat when exercising very hard. Other
cetaceans migrate regularly between polar and tropical waters. Many pinnipeds haul out
onto beaches where it is very hot, but spend considerable amounts of time in very cold
waters. The polar pinnipeds encounter the reverse, where temperatures in air are dramatically lower than the cold water. A suite of physiological, anatomical, and biochemical tools,
unique to each species in some detail, enables them to maintain their body temperatures
in the face of these many environmental changes. In this section, we will look at the basic
principles of heat flow and discuss how they apply to marine mammal thermoregulation
in general, as well as some species-specific details.
Like almost all of their terrestrial relatives, marine mammals are endothermic
h omeotherms. That is, they generate their own heat and hold a constant body temperature
well above that of the environment. These groups were called warm blooded in older literature, to distinguish them from the cold blooded reptiles, amphibians, and so on. In modern
terminology, lizards, snakes, fish, and other groups are considered ectothermic heterotherms.
This means that their body temperature mostly fluctuates with their surrounding environment. A few groups cross these lines, such as some of the large, warm-bodied tunas and
sharks. They are considered ectothermic homeotherms, in that they generate and conserve a
large amount of their own heat in some areas of their bodies. The true mammalian hibernators (e.g., Arctic ground squirrels; Spermophilus parryii) are endothermic heterotherms that
let their body temperature fluctuate widely under specific conditions, but generate their
own heat to warm up.
9.2 Basics of heat flow
The basic equation of heat balance involves moving heat energy through conduction,
convection, radiation, and evaporation. In conduction, heat flows from a warm body that is
in contact with a colder body following the principles of the Fick equation. The rate of heat
transfer is determined by the area of contact, the thickness of the surface that the heat must
travel through, the nature of the material in contact with the object, and the temperature
difference between the warm and colder bodies as below:
Rate
kAT T
thickness
2
1
=
-
where the rate of heat flow is directly proportional to the thermal conductivity of the material (k; cal s −1 cm −1 °C −1 ), area of contact (A; cm 2 ) normal to the heat flow direction, the
temperature difference (T 1 − T 2 ; °C) between the two bodies, and inversely proportional to
the thickness of the material (cm). One can visualize this by thinking about being outside
on a cold day with a warm, down jacket. We know that our down jacket with its great
insulation capacity works better than a thin, cotton jacket. Similarly, if we can curl up into
a smaller size, we will stay warmer. All of these are practical aspects of the Fick equation
describing heat flow.
Consider each of the Fick variables: Conduction is the direct contact transfer of heat energy from
a warm body to a cooler body. A common misperception is that cold flows from the cold
to the warm body. Energy can only move down a gradient, not up. For marine mammals,
this is heat that will be lost to any contact material that is cooler than their external body
Marine Mammal Physiology: Requisites for Ocean Living
versus dissipate heat varies widely given the global distribution of this large group of
mammals. For instance, some cetaceans spend their entire lives in ice-laden polar waters,
and therefore would only need to disperse excess heat when exercising very hard. Other
cetaceans migrate regularly between polar and tropical waters. Many pinnipeds haul out
onto beaches where it is very hot, but spend considerable amounts of time in very cold
waters. The polar pinnipeds encounter the reverse, where temperatures in air are dramatically lower than the cold water. A suite of physiological, anatomical, and biochemical tools,
unique to each species in some detail, enables them to maintain their body temperatures
in the face of these many environmental changes. In this section, we will look at the basic
principles of heat flow and discuss how they apply to marine mammal thermoregulation
in general, as well as some species-specific details.
Like almost all of their terrestrial relatives, marine mammals are endothermic
h omeotherms. That is, they generate their own heat and hold a constant body temperature
well above that of the environment. These groups were called warm blooded in older literature, to distinguish them from the cold blooded reptiles, amphibians, and so on. In modern
terminology, lizards, snakes, fish, and other groups are considered ectothermic heterotherms.
This means that their body temperature mostly fluctuates with their surrounding environment. A few groups cross these lines, such as some of the large, warm-bodied tunas and
sharks. They are considered ectothermic homeotherms, in that they generate and conserve a
large amount of their own heat in some areas of their bodies. The true mammalian hibernators (e.g., Arctic ground squirrels; Spermophilus parryii) are endothermic heterotherms that
let their body temperature fluctuate widely under specific conditions, but generate their
own heat to warm up.
9.2 Basics of heat flow
The basic equation of heat balance involves moving heat energy through conduction,
convection, radiation, and evaporation. In conduction, heat flows from a warm body that is
in contact with a colder body following the principles of the Fick equation. The rate of heat
transfer is determined by the area of contact, the thickness of the surface that the heat must
travel through, the nature of the material in contact with the object, and the temperature
difference between the warm and colder bodies as below:
Rate
kAT T
thickness
2
1
=
-
where the rate of heat flow is directly proportional to the thermal conductivity of the material (k; cal s −1 cm −1 °C −1 ), area of contact (A; cm 2 ) normal to the heat flow direction, the
temperature difference (T 1 − T 2 ; °C) between the two bodies, and inversely proportional to
the thickness of the material (cm). One can visualize this by thinking about being outside
on a cold day with a warm, down jacket. We know that our down jacket with its great
insulation capacity works better than a thin, cotton jacket. Similarly, if we can curl up into
a smaller size, we will stay warmer. All of these are practical aspects of the Fick equation
describing heat flow.
Consider each of the Fick variables: Conduction is the direct contact transfer of heat energy from
a warm body to a cooler body. A common misperception is that cold flows from the cold
to the warm body. Energy can only move down a gradient, not up. For marine mammals,
this is heat that will be lost to any contact material that is cooler than their external body